Synonyms

Biostatus and Distribution

This adventive fly from the Pacific Region and old world tropics is found in the North Island and Nelson in the South Island of New Zealand. It is a leafminer of herbaceous brassicas (Cruciferae or Brassicae) and plants with similar sulphur compounds. In New Zealand it occurs on some garden plants, watercress and weeds.

Life Stages and Annual Cycle

The fly breeds from late spring to the autumn. It overwinters as puparia. There are probably 3-4 generations per year in Auckland.

Adult flyThe flies are small, about 2.5 mm long, smaller than vinegar flies, Drosophila, that are seen around rotting fruit. The fly is coloured is black and yellow. The head is yellow between the eyes and on the underside. The top of the mesonotum (first visible segment after the head) is shiny black with yellow down the sides. The scutellum (last part of the thorax) is mostly yellow on top, while the abdomen is black on top and yellowish brown on the underside. The underside of the thorax is black. The head is mostly yellow between the red compound eyes. The short antennae are yellow. It is a typical fly, having one pair of wings. The hind pair of wings is reduced to two small knobs, or halteres, which help the fly to balance during flight. These are yellow. The first segments of the legs are yellow while the other segments are dark grey or black. The male has rounded black external genitalia at the end of the abdomen, while the female has a dark slender end containing an ovipositor. The ovipositor is used to make holes in host plant leaves into which eggs are laid. Females also feed on leaf sap from holes in leaves made with their ovipositor.

Eggs and larvaeSingle eggs are inserted into the upper side or underside of leaves. Newly hatched larvae tunnel into leaves making serpentine mines that are visible on one side of the leaf, usually the upper side. The larvae feed on the internal cells of the leaf. They have a single black jaw which is moved from side-to-side, scraping the plant cells at the head of the mine. The plant cells are ingested and the dark green faeces excreted into the mine behind it, usually in the middle of the mine. The larva moults, or changes skin, as it gets larger. There are three larval stages (instars). The last larval instar is yellow and when fully grown is about 2.5-3.0 mm long.

The mine starts where the egg is laid and meanders over the leaf. The mine gradually widens. The mature larva cuts a slit in the epidermis (skin of the leaf) and drops to the ground to pupate in the soil or litter.

PupaThe larva pupates inside its larval skin, which turns pale brown and hard. This structure is called a puparium. The puparium is pale brown. It has a pair of stigmata (organs for breathing) at each end of the body. Shortly before the fly emerges the eyes and bristles of the adult fly can be seen through the skin of the puparium.

Fly emergenceWhen ready to emerge, part of the head, just above the antennae, balloons out. This structure, the ptilinum, pushes the front of the pupa open. There is a line of weakness between the top and bottom halves of the first three-and-a-half segments that splits allowing the top and bottom to open up. After the fly has crawled out of the puparium, the ptilinum retracts into the head, the wings expand, and the body hardens. Over the next 12 hours the fly acquires its full body colour.

It is not known how the male and females of this species find each other for mating.

Recognition

These small yellow and black flies require expert knowledge for identification. Larvae of three species of fly form leaf mines in brassicas and related plants. Flies reared from these mines and larvae found in the leaf mines can be identified. The adults of the two species of Liriomyza that make mines in brassica leaves are small, black and yellow flies. The cabbage leafminer, Liriomyza brassicae, has a shiny black mesonotum (the first visible body segment behind the head, and the first segments of the legs are yellow, while the legs of the New Zealand cress leafminer, Liriomyza watti are all black except for a very small pale area on the ‘knees’ of the first pair of legs. The mesonotum is dull black and on the scutellum (the next segment of the thorax) the area of yellow is narrower and the lateral black margins are more obvious.

Adults of the turnip leafminer, Scaptomyza flava (Fallen, 1823) (Diptera: Drosophilidae), are yellow-brown and larger, about the same size as vinegar flies, Drosophila species. (Drosophilidae).

The larvae of the two species of Liriomyza form serpentine leaf mines, whereas the turnip leafminer makes a narrow mine that expands into a blotch. The turnip leafminer larva has a compact mandibular skeleton, while the two Liriomyza species have a thin curved armature. The mandibular skeletons can be seen when the leaf mines are examined with transmitted light. To distinguish between the two Liriomyza species, the larvae need to be exposed. The larvae of the cabbage leafminer are yellow, while those of the New Zealand cress leafminer are white.

Natural Enemies

PredatorsThere are no reports of predators of the flies or puparia, but it is likely that they are preyed upon by birds, spiders and predatory insects. Some female wasp parasitoids feed on leafminer fly larvae as well as parasitising fly larvae.

ParasitoidsTwo species of parasitoid wasps that were reared from larvae and puparia of cabbage leafminers in New Zealand have been identified. One wasp parasitoid, Opius cinerariae (Hymenoptera: Braconidae), lives in the fly larva and emerges from the fly puparium. A second parasitoid, an unnamed species, Proacrias n.sp. (Hymenoptera: Eulophidae) kills the fly larva and pupates in the leaf mine. There may be other parasitoids of this leafminer in New Zealand. Several species in New Zealand are known to attack cabbage leafminers in other countries.

Host Plants

The cabbage leafminer lives in herbaceous brassicas (Cruciferae or Brassicae) and plants with similar sulphur compounds. In New Zealand it occurs on some garden plants, watercress and weeds. In other countries, host plants include peas, Pisum sativum (Leguminosae).

The adult female fly makes small punctures in young leaves for egg laying and for feeding, but these do not appear to mark the leaf. The larva of the cabbage leafminer burrows through the leaf, making mines that are visible on one or both sides of the leaf. The mine meanders over the leaf gradually widening.

Control

In some other countries the cabbage leafminer is a serious pest of brassicas such as cabbage. However, the strain of fly in New Zealand is mostly found in brassica weeds and some ornamental plants. The cabbage leafminer and two other fly leaf miners have been found in watercress, Nasturtium officinale (Cruciferae), but have not been reported causing problems in commercial crops.

In gardens, the three species form mines in garden nasturtium, Tropaeolum majus (Tropaeolaceae).

In summer the populations of the two Liriomyza species are reduced by parasitoids.